SAN FRANCISCO - Dugongs, by all accounts, are gentle creatures, largely content to munch sea grass on the bottom of ocean shallows.

Once upon a time, dugongs allegedly fooled what were presumably desperately lonely sailors into mistaking them for mermaids. Now, according to environmentalists, at least some varieties of these huge relatives of the manatee are themselves in danger of disappearing into mythology.

Of particular concern is the Okinawa dugong, a subspecies that may include as few as 50 animals, according to a lawsuit filed last fall in U.S. District Court in San Francisco. The Okinawa variety is part of a larger population of dugong listed as endangered under the U.S. Endangered Species Act.

Dugongs can live as long as 70 years and grow to nearly 1,000 pounds. Most of the world's population lives in northern Australian waters.

That the Okinawa dugong survives at all may be something of a miracle. It inhabits waters off one of the bloodiest pieces of real estate on earth.

More than 200,000 people - half of them Japanese soldiers and the remainder civilians - died on Okinawa in the spring of 1945, making the last major battle of World War II's Pacific campaign also the deadliest. More people may have died there than in Hiroshima and Nagasaki a few months later.

American military casualties included more than 40,000, including an estimated 12,000 killed, many as a result of a hurricane of kamikaze attacks. Japan in 1945 considered Okinawa - about 400 miles south of the country's main islands - the front line of its home defense. The horrific nature of the battle may have helped convince American war planners to drop the atom bombs rather than invade Japan with conventional forces.

Okinawa was returned to Japanese control in 1972 but the American military maintains dozens of bases there, including about three-quarters of the U.S. forces assigned to Japan.

Having survived the storm of fire and steel, the dugong nevertheless continue to be threatened by military operations on Okinawa, according to those interested in the animals' preservation.

Of particular concern are proposals to move the U.S. Marine Corps' Futenma air base from its current cramped location on one part of the island, where it is surrounded by civilian development, to an offshore site. The base, which supports helicopters and fixed-wing aircraft, is home to more than 4,000 Marines and sailors.

The primary problem for environmentalists is that the new home of the airfield would be literally right on top of and next to a coral reef. The reef area provides "the most important remaining habitat" for the rare dugong, according to the lawsuit.

To block the Futenma project, a coalition of American and Japanese environmentalists turned to an unusual source of legal firepower, the National Historic Preservation Act. That law, unlike traditional environmental statutes, requires compliance by the U.S. government in its overseas activities, according to attorneys representing the groups.

The NHPA covers "any property listed on the host country's equivalent of the National Register of Historic Places," said Marcello Mollo, a lawyer for Oakland's nonprofit Earthjustice law firm.

The Okinawa dugong is a protected "national monument" under the Japanese "Law for the Protection of Cultural Properties," which is the equivalent of the historic register, according to the lawsuit, Okinawa Dugong v. Rumsfeld, 03-4350MHP.

The lawsuit seeks a court order setting aside plans to build the base until studies are conducted to find out how to avoid harming the animals.

And so the same law that might be used to preserve a Civil War battlefield in Virginia or a saloon in Oakland could be instrumental in saving the dugong. Mollo said it was necessary to turn to the historic preservation law because of the American military's general reluctance to abide by foreign countries' environmental statutes.

Attorneys for the government declined to comment about the case. However, at a hearing before U.S. District Judge Marilyn Hall Patel in March, Justice Department lawyer Charles Shockey said any actual development at the site is years away.

Okinawa Dugong

"There is no project anticipated any time soon," Shockey said, adding that building the new facility on or near the coral reef is only one of the proposals under consideration.

Documents from the Japanese Ministry of Foreign Affairs show that at least by 1996 the construction of a "sea-based facility" to handle the air base's helicopters had support from ranking U.S. and Japanese officials. The offshore strip was considered "the best option in terms of enhanced safety and quality of life for the Okinawan people while maintaining operational capabilities of U.S. forces," according to the report.

The lawsuit suggests plans for the new facility are far along, and Mollo told the judge they call for an 8,000-foot runway that could accommodate helicopters and airplanes.

Also in the works are designs for subsurface seismic testing that could disturb the animals, Mollo said.

More about the government's plans could be released next month, when the Department of Defense is scheduled to file a motion to dismiss the case. Patel scheduled a hearing for August to listen to arguments from both sides.

Until, then, presumably, the dugongs will go about their business, oblivious to the fact they may once again face a military assault from the United States, or that their fate may be in the hands of lawyers and judges thousands of miles away.